The present invention relates to an imaging lens for forming an image on an imaging element such as a CCD sensor and a CMOS sensor. In particular, the present invention relates to an imaging lens suitable for mounting in a relatively small camera such as a cellular phone, a digital still camera, a portable information terminal, a security camera, an onboard camera, and a network camera.
An imaging lens for mounting in the above-described small-sized camera is required to have a small size, as well as sufficient optical performances that can be compatible with recent imaging elements with high resolution. Conventionally, when an imaging lens did not have high resolution, it was possible to attain both sufficient optical performances suitable for resolution of the imaging element and miniaturization even with an imaging lens having a two-lens or three-lens configuration. However, as resolution of an imaging lens becomes higher, required optical performances become higher, so that it becomes difficult to attain both sufficient optical performances with satisfactorily corrected aberration and miniaturization.
For this reason, there have been studies in adding another lens, i.e. an imaging lens with a four-lens configuration. For example, an imaging lens described in Patent Reference includes in this order from an object side, a first lens that has a convex shape on the object side and is positive; a second lens that has a shape of a negative meniscus lens directing a concave surface thereof to the object side; a third lens that has a shape of a positive meniscus lens directing a convex surface thereof to the object side; and a fourth lens that has a shape of a positive meniscus lens directing a convex surface thereof to the object side. According to this configuration, satisfactory optical performances are obtained while restraining increase of the total length of the imaging lens by setting preferred ranges for a ratio of a focal length of each of the first to the third lenses to a focal length of the lens system, for refractive index of the first lens, and for Abbe's number of the first lens, and then respectively keeping those values within the ranges.
Patent Reference: Japanese Patent Publication No. 2007-122007
According to the imaging lens described in Patent Reference, it is possible to attain relatively satisfactory aberrations. In each year, however, there have been advancements in miniaturization and performances of devices themselves for mounting the above-described small-sized cameras, so that the size required for such imaging lens has been even smaller than before. In case of the lens configuration described in Patent Reference, it is difficult to attain both miniaturization and satisfactory aberration correction, so as to meet the above-described requirements.
In view of the problems of the conventional techniques described above, an object of the present invention is to provide an imaging lens that can satisfactorily correct aberrations in spite of a small size thereof.